Lens for electron beam recorder



Dec. 9, 1969 R. s. BERGLUND 3,483,427

LENS FOR ELECTRON BEAM RECORDER Filed Nov. Z5. 1967 VENTOR.

United States Patent O 3,483,427 LENS FOR ELECTRON BEAM RECORDER Robert S. Berglund, North Hudson, Wis., assignor to Minnesota'Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed Nov. 3, 1967, Ser. No. 680,437 Int. Cl. H01j 29/56 U.S. Cl. 315-31 11 Claims ABSTRACT F THE DISCLOSURE An electron beam recorder wherein two focusing coils are positioned for varying the current density and the cross-sectional area of the electron beam, an apertu-red plate is positioned between the focusing coils for passing only the central portion of the electron beam, and a programmed electrical circuit is operatively coupled to the electron beam source and the focusing coils for selectively varying the electron beam current, current density and cross-sectional area in order to maintain at the recording surface a predetermined current density of electron beams having differing cross-sectional areas.

Electron beam recorders for imaging characters on an electron sensitive media are known. In certain electron beam recorders, characters of different sizes may be recorded. Position or addressing of an electron beam onto =a particular location on the media is lprovided by known deflection circuitry. When small size characters are recorded on an electron sensitive media, a single focusing device focuses the electron beam to a single predetermined cross-sectional area or spot size. When single characters are recorded, the current density of the electron beam at a predetermined spot size is maintained at a predetermined density by any one of several known techniques.

When large size characters are recorded on an electron sensitive media, the electron beam is deflected over and images a larger surface of the media relative to that imaged for an electron bea-m for `a smaller character. Sufcient electron beam current density for imaging the media is obtained by increasing the electron beam current from an electron gun source. For example, when the electron beam is to record a character which is doubled in size, the electron beam cross-sectional area or spotsize and current density should increase four-fold. Such an increase in both spot size and density is necessary to maintain a uniform relationship between line width and character size of the recorded images.

When the electron beam current produced by the electron gun is substantially increased in current density, say by a factor of four, certain inherent problems are encountered. For example, if a filament is operated at a higher emission level, it is usually necessary to increase the filament temperature which in turn shortens fila-ment life. Filament life is inversely proportional to filament emission density. Thus, a filament operated `at `a higher filament temperature :and relatively high emission density usually must be replaced at more frequent intervals than filaments operated at a lower temperature and lower emission density. Another inherent problem of operating the filament at higher temperatures is that the increased amount of thermal energy must be dissipated by the electron gun. Further, the power supply and associated circuitry must be designed to operate at higher current levels.

It is also known in the prior art to utilize a plurality of focusing lens, such as for example U.S. Patent No. 2,898,467. The focusing lens reduce the electron beam spot size or cross-sectional `area to a diameter of less lthan 2p. to produce :and maintain an extremell fine writing 3,483,427 Patented Dec. 9, 1969 spot for recording oscillographic curves on a photographic media.

The present invention discloses a unique and novel apparatus and method for using at least two focusing lens for selectively varying an electron beam spot size or cross-sectional area while maintaining the electron beam current density at substantially a predetermined current density. In one embodiment of the present invention, the t-wo focusing means are utilized `for imaging an electron sensitive recording media with characters of two different sizes. By using the teachings of the present invention, the electron beam cross-sectional area at the surface of the media can be selectively increased four-fold while the electron beam `current need be increased only two-fold.

One advantage of the present invention is that the electron beam spot size or cross-sectional area can be electrically switched between two predetermined cross-sectional areas `at high switching rates.

Another advantage is Ithat an electron beam recorder utilizing the teachings of this invention can image an electron sensitive media with characters having a plurality of sizes.

A further advantage of the present invention is that a new method is taught for selectively varying the crosssectional area of an electron beam while maintaining the current density thereof 4at substantially a predetermined density.

Another advantage of the present invention is that apparatus is disclosed which is responsive to digital signals for recording, with :an electron beam on `an electron sensitive media, characters of at least two sizes.

These and other advantages will become apparent when considered in light of the following detailed description of a preferred embodiment taken together with the drawing wherein:

FIGURE 1 is a pictorial representation partially in block form of an electron beam recorder utilizing the teachings of the present invention;

FIGURE 2 is a diagrammatic representation of the electron beam path for recording characters of small size on an electron sensitive media with an electron beam having a spot size in the order of 15a;

FIGURE 3 is a diagrammatic representation of the electron beam path for recording characters of large size on an electron sensitive media with an electron beam having a spot size in the order of 30a;

FIGURE 4 is an isometric view partially in section illustrating a grid-filament assembly capable of being used in the electron beam recorder of FIGURE l; and

FIGURE 5 is an isometric view partially in section illustrating a stigmator capable of being used in the electron beam recorder of FIGURE l.

Briefly, the electron beam recorder, apparatus and method of the present invention includes in combination a means for discretely controlling the cross-sectional area of a modulated electron beam while maintaining a substantially uniform current density for imaging an electron sensitive media. In one embodiment, a vacuum chamber has at one end thereof an electron gun for generating an electron beam. Located at the other end of the chamber is an electron sensitive media which is adapted to be imaged by the electron beam and the media is positioned along a predetermined path in alignment with the electron gun. Means are provided for defining an aperture and the aperture defining means is positioned along the predetermined path for passing substantially the central portion of the electron beam to form an electron beam of uniform diameter. A first focusing means is positioned along the predetermined path and relative to the means defining an aperture for focusing the central portion of the electron beam on the media. A second focusing means is positioned adjacent the electron gun and is capable of being selectively energized for focusing the electron beam to a smaller uniform diameter. The first focusing means is operative to focus the central portion of the electron beam at a first predetermined cross-Sectional area and predetermined density on the media when the second fouesiug means is de-energized. Also, the first focusing means is operable to focus the central portion of the electron beam at a larger second predetermined crosssectional area when the second focusing means is selectively energized. When the rst focusing means focuses the central portion of the electron beam at the second predetermined cross-sectional area, the current density thereof can be made substantially equal to the predetermined current density by doubling the electron beam current each time the focused electron beam cross-sectional area is doubled.

FIGURE 1 illustrates, in a pictorial representation, an electron beam recorder incorporating the teachings of the present invention. The electron beam recorder includes an electron gun chamber, generally designated as 10, which terminates in an imaging aperture 12. A focused electron beam 14 is deflected onto and images an electron sensitive media 16. The electron sensitive media 16 is positioned in communication with the imaging aperture 12 through a slot-type vacuum seal, generally designated as 20.

A vacuum seal adapted for use with an electron beam recorder is described in a copending application filed Nov. 3, 1967, U.S. Ser. No. 684,096, entitled Corpuscular Beam Recorder invented by Earl K. Hoyne. In one ernbodiment, the electron sensitive media was selected to be dry silver film having a Mylar backing such as that described in Belgium Patent No. 663,112. Alternatively, the photographic film could be a high resolution, grain-free spectroscopic film, such as Kodak type 649-GH.

The electron gun chamber 10 has an electron gun, generally designated as 22, at one end thereof. At the other end of the electron gun chamber 10 and generally in alignment with the electron gun 22 is the imaging aperture 12. The electron gun chamber 10 is maintained at a predetermined vacuum level, such as for example l5 torr, by means of a Vacuum pump (not shown) operatively connected to the electron gun chamber through conduit 26. The vacuum within the electron gun chamber 10 can be maintained during loading of the photographic media by use of a flap valve 28.

The electron gun 22, in one embodiment, comprises a removable filament-grid assembly 30. The filiment-grid assembly includes a filament 34 which is supported by support conductors 36. The filament 34 is supported within a grid 38. The grid 38 is in the form of a solid spherical member having an aperture therethrough into which the filament 34 is inserted. The outer surface of the grid 38 is in the form of a concave spherical-shaped surface 40. The filament-grid assembly 30 is illustrated in greater detail in FIGURE 4. A mating cylindrical anode 44 is positioned a predetermined distance from the spherical surface 40 of grid 38.

The anode 44 forms an aperture 46 therein through which an electron beam passes. Within the interior of anode 44, a focusing slot 50 having a focusing aperture receives electrons emitted by filament 34 which have passed through the aperture 46. The anode 44 and slot 50 are threaded and supported by a threaded support member 52.

The filament and grid are energized by a high voltage filament and grid power supply 56. In one embodiment, the filament is maintained at a voltage of about -21 kv., the grid at about 21.3 kv. and the anode 44 at ground potential. In this embodiment, the anode 44 is spaced about .45 inch (about l2 mm.) from the grid 38. The aperture in the grid 38 is about .1 inch (about 2.5 mm), the anode aperture 46 is about .035 inch (about 1 mm.) and the aperture in the slot 50 is about .0015 inch (about 375g).

Electrons from the filament 34 are directed through the aperture 46 by an accelerating field existing between the grid 38 and anode 44. The electrons from the filament 34 are substantially directed to a point at the focusing aperture of slot 50. The electron beam emanates from the focusing aperture of slot 50 through the support member S2 and spreads into a conical-shaped beam 58 as it passes down the center of the electron gun chamber 10.

It is contemplated that the present invention could be utilized in combination with electron guns which produce the electron .beam in discrete patterns rather than in a spot. For example, the electron beam can be used in a character generator electron gun for generating a predetermined character. In particular, the electron beam can be used to flood a mask having the character formed therein such that the electron beam passing therethrough has a cross-sectional size which is in the form of the character mask.

Means defining an aperture, such as for example an aperture plate 60, is positioned at any one of many desired locations at a predetermined distance from the anode 44. The aperture plate 60 has an aperture therethrough, generally designated as 62. The central portion of the conical-shaped electron beam 58 passes through aperture 62 and is formed into an electron beam of a desired diameter. The central portion of the electron beam 58 is focused into a focused electron beam 14 which is ultimately used to image the electron sensitive media 16.

A first focusing means, such as for example a magnetic focusing means 66, is positioned relative to the aperture plate 60 between the aperture 62 and the media 16. The magnetic focusing means 66 focuses the central portion of the electron beam 58 into a thin focused electron beam 14 having a predetermined spot size. In one ernbodiment, the aperture 62 is selected to have a diameter of about .1 inch (about 2500,10 and is preferably positioned just before the magnetic focusing means 66. The magnetic focusing means 66 ultimately focuses the electron beam to a cross-sectional area of 15u which electron beam may have a predetermined current density in the order of about .5 amp/cm.2 to about 1 amp/cm?.

A stigmator `68 is used to correct for irregularities in beam cross-sectional area. A deflectron 70 is used to stroke or selectively deflect the electron beam 14 in a predetermined character pattern which is ultimately imaged on the electron sensitive media.

A dellecting means, such as for example a magnetic deflection coil 72, is used to address or position the focused electron beam 14 onto a predetermined area of the electron sensitive media 16. The deflectron 70 then selectively dellects or strokes the electron beam in small discrete strokes to image a character onto the predetermined area on the media 16.

A second focusing means, such as for example magnetic focusing means 80, is positioned adjacent the anode 44 and along the predetermined path between the electron gun 22 and the media 16. The magnetic focusing means 80 when energized causes the conical-shaped electron beam 58 to be focused to a smaller uniform diameter at the aperture plate 60.

A direct current centering means, such as for example a centering coil 82, is used to center the electron beam 58 onto the aperture plate 60.

The magnetic focusing means 66 is controlled by a focus coil power supply 88. The stigmator 68 is energized by a stigmator power supply 90 and by proper adjustment of the stigmator 68 the cross-section of the electron beam can be made to be nearly circular in cross-section. The vdeflectron 70 for generating a character is controlled by deflectron power supply 92. The deflection coil 72, for addressing the electron lbeam 14 to a predetermined position on the media 16, is supplied from a magnetic deflection amplifier 94. The magnetic focusing means is energized and controlled by a focus coil power supply 96. The

S direct current centering coil 82 is controlled by a current regulated power supply 98 which maintains both the horizontal and vertical centering position of the electron beam 58 after the same has been selectively adjusted onto the aperture plate 60.

In one embodiment, the electron beam recorder was used as a device for converting information from a magnetic tape into a permanent record onto a photographic film. Alternatively, the electron beam recorder could be connected directly to a computer as an output device. In any event, the output from either a magnetic tape or a computer, designated :by box 100, is in a digital format. The digital signals appear on an input 102 and are applied to a controller and memory unit 104. The controller and memory unit 104 in turn controls various components on the electron beam recorder to record information onto the electron sensitive media 16 in the form of characters of a predetermined size.

The cross-section of the focused electron beam 14 is discretely controlled for generating and recording characters of a predetermined size onto the electron sensitive media 16. 'Ihe controller and memory unit 104 initially addresses the focused electron beam 14 to a predetermined position on the electron sensitive media 16. A programmed direct current -voltage is applied to the deflection amplifier 94. The amplifier 94 in turn applies a deection current to coil 72 to deflect the focused electron beam 14 onto the addressed location on the media 16. If the character to be recorded is of a small size, the controller and memory unit 104 de-energizes the power to the focus coil power supply 96 disabling the magnetic focusing means 80. The focus coil power supply 88 to the magnetic focusing means 66 is set at a predetermined level focusing the electron beam 14 to a small diameter, such as for example a 15p. diameter. Thereafter, the controller and memory unit 104 programs the deectron power supply 92 with electrical signals representing a character pattern. The electrical signals, in the form of discrete voltage levels, are applied to deflectron 70 which establishes an electrostatic field for defiecting the focused electron beam 14. The deflected electron beam generates a character, such as for example a graphic sym-bol, a letter or a number, which is subsequently imaged onto electron sensitive media 16 at the addressed position of the focused electron beam 14. In one application, the electron beam having a 15u cross-sectional area is capable of generating a 120,1. character with an electron beam current density of about .5 amp/ cm?. Also, since the electron beam current density can be doubled for recording largersize characters, if desired the electron beam density at the small character size can be doubled to about l amp/ c-m.2 without increasing the electron beam spot size for imaging a character on the electron sensitive media at a higher density.

If the controller and memory unit .104 determines that a large character is to be recorded on the media 16, certain changes are electrically made in the electron beam recorder. Specifically, the focus coil power supply 96 is energized causing Athe magnetic focusing means 80 to decrease the cross-sectional diameter of the conicalshaped electron beam 58. Also, if desired, the focus coil power supply 88 can be slightly decreased in power which slightly decreases the focusing action of the magnetic focusing means 66.

The controller and memory unit 104 then programs the deectron power supply 92 and subsequently the deectron 70 to generate the desired large character. Concurrently, the controller and memory unit 104 proygrams the high voltage, filament and grid power supply 56 to increase the current level of the electron beam produced from the filament 34.

In one embodiment, the beam cross-sectional diameter of focused electron beam 14 was increased from 15p to 30p thereby increasing the beam diameter two-fold. When the magnetic focusing means 80 is energized, the beam current density appearing at aperture 62 is substantially increased in magnitude. Similarly, by doubling the current density of the electrons produced in filament 34, the resulting cross-sectional diameter of focused electron beam 14 has a cross-sectional area which is approximately four times as large as that utilized for a small character and at substantially the same predetermined current density as the electron beam for the small character. However, the current level of the electron beam from the electron gun is only doubled in magnitude even though approximately four times the current density Was required at the surface of the electron sensitive media 16. In one application, the electron beam having a 30u crosssectional area is capable of generating a 240;; character with an electron current density of about .5 amp/cm?. FIGURES 2 and 3 are diagrammatic representations of the focusing action of the lens illustrated in FIGURE 1 for maintaining the electron beam current density at substantially the same level for various beam diameters.

In FIGURES 2 and 3, a filament 110 produces electrons 1.12 which are directed by means of a grid 114 through an anode 116 onto a slotted member 118 having an opening therein which permits electrons to pass therethrough which form an electron beam of uniform diameter. An aperture plate, designated by 120, has an opening therein, designated as 122. The first focusing means is designated as 124 and the second focusing means is designated as 126. In one embodiment, the aperture plate 120 is positioned approximately 14.75 inches (about 37.5 mm.) from the opening in the slotted member 118. The first focusing means 124 is positioned approximately 17.125 inches (43.5 mm.) from the slotted member 118. The recording media 128 is positioned approximately 6.75 inches (about 17 mm.) from the first focusing means 124. In this embodiment, each of the focusing means 124 and 126 includes a magnetic focusing coil. The focusing coil for focusing means 124 has a focal length which varies between 4.3 and 4.9 and the focusing coil for focusing means 126 has a focal length of 7. The spherical aberration constant of each of the lens is selected to be as small as possible and in this embodiment was 147.

FIGURE 2 illustrates the electron beam characteristics for generating a small character. The electrons 112 passing through the slotted member 118 diverge into a conical-shaped electron beam 130 which oods or bombards the aperture plate y120. In the above-noted embodiment, the electron beam cross-section is about .25 inch (about 6250u) at the aperture plate 120. The opening y122 in this embodiment is about .l inch (about 2500p) and the focused electron beam 132 is focused -by the first focusing means 124 to a cross-section of about 15p. at the surface of the electron sensitive media 128 to image thereon a character about 120e in size. The object of the first focusing means 124 is substantially the electrons in the opening of the slotted member 118.

FIGURE 3 illustrates the electron beam characteristics for generating a large character. The second focusing means 126 is energized to focus the beam to a cross-sectional of about .1 inch (about Z500/l) at the aperture plate 120. The first focusing means .124 may be, but need not necessarily be, slightly reduced in power, such as for example a decrease in power level of less than ve percent. The focused electron beam 132 then has a crosssection in the order of 30p at the surface of the electron sensitive media 128. The object for the first focusing means 124 is a virtual image which appears to be approximately 3.5 inches (about 8.75 mm.) behind the slotted member 118 at approximately the location illustrated by dashed line 136. The virtual image for this lens is in the order of two times that of the image which appears at the opening in slotted member 118.

Concurrently, when the second focusing means 126 is energized focusing the conical-shaped electron beam 130, the grid bias of grid .114 is adjusted such that the density of electrons from filament 110 reaching the anode 116 is doubled for approximately a four-fold increase in the cross-sectional area of the focused electron beam 132. By use of the second focusing means 126 to slightly focus the conical-shaped electron beam 130 in FIGURE 3 to a smaller beam cross-section, a larger portion of the electron vbeam current density at the original power level is used. The additional electron current density is obtained by adjusting grid bias. In this manner, the current density of the electron beam at the larger cross-sectional area of, say, 30p is substantially the same predetermined current density as the electron beam for the p spot.

FIGURE 4 is an isometric representation of the filament-grid assembly of FIGURE l partially in crosssection. The grid 38 is a cylindrical-shaped grid constructed of a nonmagnetic stainless steel, such as f-or example #302 or #304. The grid 38 has an aperture 140 therethrough along the center axis thereof which terminates in and communicates with a grid holloWed-out area 146. The other end of the grid 38 terminates in a spherical concave inner surface 40 which communicates `with the aperture 140. The filament 34 can be constructed from a relatively thin tungsten wire which is electrically connected to a pair of conductors 36, which in one embodirnent were copper electrodes. The conductors 36 and filament 34 are rigidly supported by an insulating cylindrical-shaped support 142- Support 142 may be formed of an insulating material, such as Supermica 500 sold by the Mycalex Corporation of America. The support 142 has a dimension which is less than that of the grid hollowed-out area 146. Thus, the insulating sup-port 142, the

filament 34 and conductors 36 together form an integral L unit which can be located within the grid hollowed-out area 146. The filament 34 is of sufficient length to extend through the aperture 140 so as to be adjacent the spherical concave inner surface 4d. The rfilament 34 may be selectively located within the aperture below surface 4f), even with surface 40 or extending out of aperture 140 slightly beyond the surface 40.

A cover plate 148 having at least one opening for receiving the conductors 36 is secured to the grid 38 at the grid hollowed-out end 146 by means of fasteners, such as for example screws. The filament 34 can be properly aligned within the aperture 140 by means of four adjusting screws 150 which are located at 90 angles around the periphery of the grid 38.

The entire filament-grid assembly 30 is a pluggable unit which can be used in an electron gun or for an electron beam source.

FIGURE 5 is an isometric view of stigmator 68 of FIGURE l partially in cross-section. In FIGURE 5, machineable cylindrical-shaped insulating member 160, for example formed of Supermica 500, is illustrated. The member 160 has an aperture therethrough forming an inrer surface forming relatively thin outer walls 162 around the periphery of member 160. The interio-r of the cylindrical-shaped member 16() is coated with a relatively thin metallic coating, such as vapor coated, having a thickness in the order of several microns. The metallic coating on member 1160 is then scribed with a plurality of parallel lines 164 forming a plurality of parallel segments 166 which extend longitudinally to and in a spaced relationship from the longitudinal axis of the cylindricalshaped member 160. In one embodiment, eight separate segments were formed as a stigmator.

A plurality of electrical conductors 170, such as for example copper leads, are rigidly mounted into and extend through the thin outer wall 162 to be mechanically and electrically connected to the segments 166. In one embodiment, the copper leads are threaded and the cylindrical-shaped member 160 has a plurality of spaced apertures which are tapped to provide a rigid mechanical connection between the copper leads and the cylindrical shaped member 160. Thus, at least one segment 166 is electrically connected to a lead 170. Further, each segment is spaced from and electrically isolated from each other segment. Thus, each segment 166 is capable of having a predetrcmined potential applied thereto for correcting astigmatism in the electron beam cross-sectional area. The resulting stigmator is operated in a manner similar to that used for stigmators known in the art. Such a stigmator is a compact economical device which can be used in an electron beam recorder for imaging a photographic media with an electron beam,

Alternately, the stigmator cylindrical-shaped member could be formed of a standard ceramic-type material, such as Alsimag 719 manufactured by the American Lava Corporation. By utilizing known techniques, a thin copper coating can be coated onto the interior of the ceramic member in a pattern to form a plurality of electrically isolated copper segments. The copper leads can be fused onto the ceramic material and electrically connected to the copper segments.

It is apparent that certain modifications, improvements and the like can be made of the present invention and all are deemed to be within the scope of the appended claims.

What is claimed is:

1. In apparatus which is responsive to digital signals for recording with an electron beam on an electron sensitive media characters of at least two sizes, means for discretely controlling the spot-size of said electron beam` while maintaining the current density thereof at substantially a predetermined current density during imaging of said media with said characters, said means comprising: means defining an aperture positioned to intercept said electron beam to form from the central portion thereof an electron beam of a desired diameter;

a first focusing means positioned relative to said aperture defining means for focusing said central portion of said electron beam on said media, said first focusing means being selected to have a focal length wherein said central portion of said electron beam is focused to a first predetermined spot size which is capable of being deflected in a character pattern for imaging said media with a character of a first size;

'a second focusing means positioned before said first focusing means and adjacent said modulated electron beam, said second focusing means being selected -to have a focal length wherein said central portion of said electron beam is focused to a smaller dif ameter which changes the focusing of said first focusing means in a manner to increase said central portion of said electron beam to a larger second predetermined spot size which is capable of being deflected in said character pattern for imaging said media with said character at a second size; and

programming means responding to said digital signals for determining a character and the size of said character which is to be imaged on said media and for controlling deflection of said focused electron beam in said character pattern, said programming means being capable of controlling said second focusing means for producing said first predetermined spot size when said character pattern is to be imaged on said media at said first size and for producing said second predetermined spot size and increasing the electron beam current intensity to a level wherein the current density of said electron beam spot on said media at said second predetermined spot size is substantially equal to the current density at said rst predetermined spot size when said character pattern is to be imaged on Said media at said second size.

2. The apparatus of claim 1 wherein said means defining said aperture is positioned just before said first focusing means and said electron sensitive media is a photographic film.

3. The apparatus of claim 2 wherein said first focusing means and said second focusing means include magnetic focusing coils and wherein said first predetermined spot size is selected to be about 15p and said second predetermined spot size is selected to be about 30p.

4. A method for selectively varying the cross-sectional area of an electron beam on a surface between twopredetermined cross-sectional areas while maintaining the current density thereof at substantially a predetermined current density comprising the steps of:

generating an electron beam having a selected current level; n

directing said electron beam along a predetermined path;

selectively passing the central portion of said electron beam to produce an electron beam of uniform diameter;

focusing said central portion of said electron beam tc a first predetermined cross-sectional area at said predetermined current density on said surface; focusing said electron beam to a smaller uniform diameter when said electron beam cross-sectional area is to be selectively increased resulting in said central portion of said electron beam being focused to a larger second predetermined cross-sectional area on said surface;

increasing the current of said electron beam when said focused electron beam cross sectional area is at said second predetermined cross-sectional area. to a level wherein the current density thereof substantially equals said predetermined current density; and

programming said focusing of said electron beam to a smaller uniform diameter and said increasing of Isaid electron beam current level for selectively switching said electron beam on said surface between said two predetermined cross-sectional areas.

5. The method of claim 4 wherein said surface is part of an electron sensitive media further comprising the steps of:

deflecting `said focused central portion of said electron beam having a predetermined cross-sectional area onto a preselected position on said surface; and

selectively stroking said deflected electron beam in a character pattern to image a character on said media.

6. An apparatus for selectively controlling the crosssectional area of an electron beam at a surface while maintaining at the surface a predetermined current density of the electron beam comprising:

means for generating a said electron beam of variable current;

means for selectively focusing the generated electron beam to vary the current density of the electron beam;

means for passing the central portion of the focused electron beam;

means for focusing the passed central portion of the electron beam to a desired cross-sectional area at the surface; and

programming means controlling the generating means and both focusing means for selectively simultaneously increasing the current of the generated electron beam, focusing the generated electronbeam to in- 'crease current density, and focusing the passed central portion of the electron beam to a desired larger cross-sectional area at the surface, while maintaining at the surface, the predetermined current density.

7. The apparatus according to claim 6 further comprising means for dee'cting the passed central portion of the electron beam onto an addressed location on the surface.

CTI

8. The apparatus according to claim 6 further comprising means for selectively stroking the electron beam in a character pattern to image a character on the surface.

9. An electron beam recorder for recording multisize character patterns on a media surface, with an electron beam having a maintained predetermined current density at the media surface comprising:

a variable current electron beam source;

a plate defining an aperture for passing the central portion of a said electron beam to a said media surface;

a first focusing means located between the plate and the position of a said media surface for controlling at a said media surface the cross-sectional area of the passed central portion of a said electron beam having a predetermined current density at a said media f surface'; `a second focusing means located between the electron beam source and the plate for controlling the current density of a said electron beam at the aperture; and a programming circuit controlling the electron beam lsource and the first and second focusing means for selectively simultaneously increasing electron beam current, operating the second focusing means to incre'ase the current density of the electron beam at the aperture and operating the first focusing means to provide at a said media surface a desired larger electron beam cross-sectional area and the predetermined electron beam current density.

10. The electron beam recorder according to claim 9 further comprising deflection means located between the first focusing means and the position of a said media surface and operatively coupled to the programming circuit for defiecting the passed central portion of the electron beam to an addressed location on a said media surface in response to a programmed signal received from the programming circuit.

11. The electron beam recorder according to claim 10 characterized by a deliectron located between the first focusing means and the position of a said surface media and operatively coupled to the programming circuit for discretely stroking the passed central portion of the electron beam in a character pattern in response to a signal representative of a character pattern received by the programming circuit to image the media surface with a said character pattern at the addressed location.

References Cited UNlTED STATES PATENTS 2,422,807 6/1947 Smith 315-3] X 2,464,396 3/ 1949 Hillier et al. 315-31 X 2,464,419 3/1949 Smith et al 315--31 X 2,627,589 2/ 1953 Ellis 315-31 2,640,950 6/1953 Cook 315-31 2,781,171 2/1957 Hagen 313-78 X 3,358,174- 12/1967 Glenn 315-31 X 3,418,520 12/1968 Barber et al. 315-31 RODNEY D. BENNETT, JR., Primary Examiner M. F. HUBLER, Assistant Examiner U.S. Cl. X.R. 313-83 

